By Brendan McKenna, Marketing Manager, Agere Systems
Designers of wireless infrastructure equipment today face a challenging technology climate: the future is looking to be an all-packet switching future, but many of today's networks and services are based on cell-based Asynchronous Transfer Mode (ATM) and Time Division Multiplexing (TDM) technology. And the infrastructure is migrating slowly. Service providers need cost-effective equipment that will support today's mainstream voice and data services based on full-featured ATM, while enabling them to transition to future multiservice offerings, such as converging onto one network ATM and Internet Protocol (IP) packet-based technologies. In this market environment, designing, developing, and delivering to market full-featured ATM capabilities has become crucial for bridging current and next-generation wireless infrastructure equipment using ATM and IP technology.
.Today's networks carry voice, video and data using separate networks. For carriers, deploying and maintaining separate parallel networks - packet networks using routers and switches for data and TDM-based circuit-switched networks for voice is expensive. But multimedia and data traffic are growing rapidly, and in many ways, this growth is spurring the convergence of data and voice networks. For example, with 3G wireless services, users are accessing the web (data) using cell phones on a network designed for voice. And in other areas, voice is being carried over IP data networks designed for best effort delivery. With these trends in mind, many carriers have their eye on a future based on an all-IP network. .
However, with the huge current infrastructure of ATM and TDM networks in place, the transition to an all-IP future is likely to take years to happen. During the transition, service providers need flexibility to provision different types of services and to address many variations in traffic mix. In addition, to maximize network efficiency, IP packets will be classified and grouped into several flows according to latency and bandwidth needs, among other factors; and delivered according to Service Level Agreements that enable service providers to enhance revenues by charging more for premium services. The key to these capabilities will lie with multi-service routers and switches. .
Next-generation wired and wireless networks will be multi-service networks that can simultaneously support multiple protocols, such as IP, ATM, multiprotocol layer switching (MPLS) and time division multiplexing. For example, voice and video traffic traditionally have been carried over ATM networks while IP carried data traffic. ATM is a connection oriented communication paradigm that segments data into fixed size packets called cells and has built-in quality of service (QoS) capabilities. The ATM Forum TM 4.1 specification defines service categories and corresponding traffic descriptors with QoS parameters that allow allocating resources based on the service category, such as voice, video, and data (Constant Bit Rate (CBR), Variable Bit Rate (VBR), Available Bit Rate (ABR) and Unspecified Bit Rate (UBR)). AAL (ATM Adaptation Layer) sits above the ATM layer and is responsible for formatting and segmenting packets into fixed sized cells, a process called SAR (Segmentation and Reassembly). Because ATM is connection-oriented, it is suitable for implementing a QoS guarantee mechanism emulating that of the conventional circuit-switched network. .
IP, on the other hand, is a connectionless technology with variable-sized packets. IP can be described as a best-effort delivery scheme with little or no traffic segregation and prioritization. To meet the delay, jitter, throughput and bandwidth guarantees required by time-critical data such as voice and video, IP networks must support provisioned service classes. Service providers have adopted solutions based on MPLS in combination with Differentiated Services (DiffServ). These scheduling and traffic management algorithms are quite different from those used by ATM. .
Design Challenges for Multi-Service Base Stations
As a result, OEM designers face several challenges surrounding multi-service systems that support QoS across multiple protocols. Three main technologies, however, stand out in today's technology environment: traffic management, SAR, and ATM. Because these technologies are complex, equipment designers need to collaborate with silicon suppliers that can address the wide range of hardware and software challenges, including multiservice support, reliability, flexibility, and comprehensive functionality, while meeting the traditional constraints of power consumption, cost, and time-to-market. .
For example, while migrating towards an all-IP future, current 3G/UMTS (Universal Mobile Telecommunication Services) networks are ATM-based and use AAL2 and/or AAL5 for user-traffic transport. AAL-2 (ATM Adaptation Layer, Type 2) is the transport technique to transport real-time, variable-bit-rate user information. Real-time, variable bit rate traffic is how compressed speech is transported across today's mobile networks with payloads that vary during the course of a conversational flow from 2 to 40 octets per 20-ms speech frame. AAL2 enables such variable-length packets to share a single ATM virtual circuit (VC), up to 240 speech sessions. AAL2 also enables mobile network service providers to avoid recurring monthly transmission service leasing charges billed on a per-VC basis, because a single VC transports many user flows. Considering the aggregate load, sharing an ATM VC across many low-speed users is more efficient and economical than, for example, dedicating individual AAL5 VC's to each user flow. .
For higher-speed flows (multimedia and web data) on an ATM network, it may be desirable to allocate an AAL5 VC to a single user flow. The reason is the load from a single flow is large enough that there is little or no sharing of bandwidth by other flows. However, AAL2 may also be used to transport medium-speed data via additional service-specific convergence sub-layer functions such as I.366.1 and I.366.2. These ITU-T standards define services for adapting higher-speed user flows into AAL2 by SS-SAR, SS-TED, and (service-specific segmentation and reassembly, service-specific transmit error detection). .
The initial specification of 3G/UMTS networks used AAL2 and AAL5 as the basis for interconnect between radio network controllers (RNC) and Node-Bs wireless infrastructure equipment. An OEM designer may further decide whether to offer support for medium- or high-speed data over AAL2. However, to align mobile networks further with IP-network evolution, subsequent releases of 3G/UMTS specifications require transport via IP-based transmission techniques. Because ATM-based mobile networks are being deployed while IP-based networks are being planned, a significant interval of time will exist where operators will see both network topology types and therefore require interworking and different provisioning schemes, including support for ATM, IP over ATM (whether AAL2 or AAL5), and all-IP, including IP over MPLS. .
Another consideration of terrestrial transport in mobile networks is the availability and cost of transmission facilities. Typically, T1/E1 links are the most ubiquitous and cost-effective services available for interconnecting RNCs to Node-Bs. Given the relatively low-bandwidth of these links, however, the need to support transport of higher traffic loads (associated with higher-speed 3G/UMTS user data flows) over low-speed links requires bundling techniques whereby multiple physical links are treated as a single logical link whose rate is the aggregate of the links. .
In ATM, inverse-multiplexing-for-ATM (IMA) is an ATM Forum-defined technique for grouping multiple T1/E1 links into a single logical link for transport rates of 6.144-Mbps (4 x T1) or 12.288-Mbps (8 x T1). This scheme avoids serialization latency because each packet may be sent over a higher-bandwidth link (i.e., the IMA group) than if only a single T1 link were used. An analogous technique for IP-based networks is multilink point-to-point protocol (ML-PPP), which bundles physical links in a similar fashion. The result is a high-speed data burst of 2 Mbps (or ~ 5,000 octets per air-frame) can be sent over an IMA bundle with little latency at comparably low-cost. Moreover, because user traffic patterns are bursty and data-subscriptions will evolve over time, incrementally deploying low-speed links commensurate with user demand is generally more cost effective to the mobile network operator than deploying a DS3 or E3 link. .
Next Generation Design Approaches
To assist their OEM designer customers in meeting the challenges of next generation multi-service equipment design, semiconductor manufacturers are migrating their product lines from discrete chips to "platforms." Platforms consist of a common set of semiconductor chips, software, electronics circuit board design and systems engineering that can be used and re-used in many types of equipment, often with only minor changes. When designers opt for a platform solution, they typically avoid the potential of higher design and debugging costs associated with using chips from multiple vendors. Designers also benefit in many other areas, gaining advantages of bundled pricing, proven reference hardware designs, and high reliability, time-tested, and reusable software code. .
The importance of software in 3G wireless infrastructure products cannot be overestimated. Software is a huge differentiator in the marketplace. It's expensive and complex to develop, maintain, and upgrade over the useful life of the hardware. Software is typically also a critical element of overall system reliability. As a result, wireless base station equipment manufacturers are well advised to look for suppliers who can deliver low-total-system-cost platforms and re-usable software, rather than just individual chips and customized, single-application-only software. .
This strategy also fits well with two other 3G wireless market trends: the need to design new equipment at ever lower costs, given the new economy; and the need for equipment manufacturers to consolidate their designs into a much smaller number of reusable platforms, thereby saving further on hardware and software development costs. Developers need to be able to write smaller, not larger, amounts of code. Less code translates to lower system costs, higher equipment reliability, and faster product deliveries. Reusing those fewer lines of code in several types of equipment with the same platform reduces system costs even more. Reusing the code in many different types of equipment is the foundation of an effective platform-based product strategy. .
A Platform for Multi-Service Network Equipment
Any platform for multi-service network equipment design must have depth and breadth of ATM features as well as robust traffic management capabilities for IP protocols. Agere Systems has developed a broad portfolio of system chips equipped with ATM and traffic management technology that are key elements of the company's platform solutions. Recently, the company announced six new semiconductor chips that dramatically improve the capacity, size, total system costs, and product delivery schedules of existing and next-generation wireless and wireline telecommunications equipment, while giving service providers opportunities for new revenues from new services. .
Two of the six new chips, APP550 (5 Gbits/s) and APP530 (2.5 Gbits/s), are network processors that perform traffic management, ATM, voice and data packet processing; operations, administration and maintenance; traffic policing; traffic shaping; buffer management; and data modification. ATM and IP interworking is enabled through on-chip AAL5 SAR functionality and a companion AAL2 coprocessor. These space-and-power-saving network processors also include an on-chip Ethernet Media Access Control (MAC) and enable telecom service providers to serve eight times more types of telecom traffic to their customers at nearly half the price of competing chips. .
The TAAD Lite and TAAD UltraLite are ATM chips that provide flexible network interface solutions for next-generation applications in which efficient transport of narrowband voice and broadband data is critical to guaranteeing network QoS. Constructed using Agere's 0.16um compound metal oxide semiconductor (CMOS) technology, these chips now integrate 12 chips into one. Each chip integrates an on-board octal framer; an IMA processor that provide for inverse multiplexing for ATM using one to four groups with two to eight links per group; cell scheduler and router; and AAL2 and AAL5 SAR functions. These chips are extremely well suited for 3G wireless base stations. This unmatched integration dramatically shrinks the size of telecom equipment, thereby reducing total system costs, simplifying product design, improving equipment reliability, and accelerating product deliveries. .
The SAR-1K and SAR-500, perform SAR functions and, like the TAAD Lite and TAAD UltraLite, target 3G wireless base stations and digital subscriber line access multiplexers. Combined, the TAAD Lite and new SAR-1K chips enable 1,000 simultaneous user channels of voice, data, or video signals. Combined, the TAAD Lite, TAAD UltraLite, SAR-1K and SAR-500 reduce overall telecom line card equipment system costs by more than 40 percent. .
Agere's Festino Wireless( platform is an example of the power of these advanced chips coupled with platform-based design. By building on this platform, designers can enjoy significant savings in development cost, schedule, and product reliability. The Festino Wireless platform also showcases the programming advantages of Agere's PayloadPlus( network processor family. Agere was able to implement a complete wireless base station network interface card (including full ATM traffic management) running on the APP530 network processor with only 1550 lines of datapath software code. While the small number of lines of code required certainly simplified initial development, the real benefit of having a much smaller code base is the reduction in lifetime cost of ownership. Having fewer lines of code slashes maintenance and evolution costs and boosts field software reliability. .
By using platforms such as Festino Wireless, designers can cost effectively support the full ATM capabilities required by today's wireless infrastructure equipment while providing a smooth and economical path to tomorrow's multiservice and packet-based network infrastructure.